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Niobrara Formation
The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member overlying the Fort Hays Limestone Member. The chalk formed from the accumulation of coccoliths from microorganisms living in what was once the Western Interior Seaway, an inland sea that divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, and pterosaurs as well as several primitive aquatic birds. The type locality for the Niobrara Chalk is Knox County in northeastern Nebraska.
==History of exploration==
The Niobrara Chalk was first studied during an expedition led by Othniel Charles Marsh of Yale University in 1870. This and following expeditions to the area in 1871 and 1872 yielded the first of many fossil vertebrate remains commonly attributed with the formation. Excavations continued through the following years up to 1879 under the direction of professional fossil collectors such as B. F. Mudge and S. W. Williston appointed by Marsh.
The Niobrara Chalk has been continuously explored ever since, with specimens being found by H. T. Martin of the University of Kansas and George F. Sternberg, the son of the famous fossil collector Charles H. Sternberg. Much of the best material from the formation is on display at the Sternberg Museum of Natural History in Hays, Kansas.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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